Relation between sequence of occurrence of the minimum air temperature and frost risk in the State of Rio Grande do Sul.

A temperatura m??nima do ar ?? uma das vari??veis meteorol??gicas que exerce relevante influ??ncia sobre as condi????es ambientais, sendo um dos fatores determinantes do crescimento e desenvolvimento das plantas. A avalia????o da temperatura m??nima do ar, na Regi??o Sul do Brasil, est?? diretamente relacionada com a identifica????o dos per??odos mais adequados para a semeadura/plantio das diversas culturas, pois permite estabelecer que as fases cr??ticas da planta coincidam com o per??odo de menor probabilidade de ocorr??ncia de temperaturas prejudiciais ?? cultura. Atrav??s do estudo de 30 anos de dados di??rios de temperatura m??nima do ar de doze esta????es meteorol??gicas de superf??cie, no Estado do Rio Grande do Sul, durante os meses do ano, foi feita a caracteriza????o dos per??odos com temperatura m??nima do ar dentro do intervalo t < 3??C. A probabilidade de a temperatura m??nima se encontrar no referido intervalo foi obtida atrav??s dos c??lculos de probabilidade condicional e da probabilidade de ocorr??ncia de dias com temperatura m??nima dentro de um per??odo de sete dias, utilizando-se o modelo da Cadeia de Markov de primeira ordem. Os resultados obtidos mostraram que a probabilidade de um determinado dia encontrar-se com uma temperatura m??nima particular, depende do estado precedente, comprovando satisfatoriamente o ajuste do Modelo da Cadeia de Markov. O estudo promoveu o mapeamento das Regi??es Ecoclim??ticas do Rio Grande do Sul com base na descri????o da seq????ncia de temperaturas m??nimas e riscos de geadas, por meio do M??todo Cadeia de Markov, do uso do modelo de regress??o linear m??ltipla e do M??todo Conrad & Pollak, identificando, dessa forma, as probabilidades das datas de ocorr??ncia da primeira e da ??ltima geada do ano, para auxiliar no planejamento e ??pocas de cultivo.The minimum air temperature is one of the meteorological variables which exercises relevant influence over environmental conditions, being one of the determining factors to the growth and development of the plants. The evaluation of the minimum air temperature, in the Southern Region of Brasil, is directly related to the identification of the more adequate periods for the sowing or planting of the various cultivation, for it permits the establishment of the critical plant phase to coincide with the period of a minor probability of harmful temperature occurrence to the cultivation. Through the thirty years of daily data study of the minimum air temperature, in twelve surface meteorological stations, in the State of Rio Grande do Sul, during the months of the year, the characterization period was done with a minimum temperature within the following interval t < 3??C. The probability of minimum temperature to be found in the referred interval was obtained through the calculation of the conditional probability and the probability of occurrence of days with the minimum temperature within an interval of seven days, using the first-order Markov Chain Model. The results obtained showed that the probability of a determined day to be found at a minimum particular temperature depends on the precedent state, confirming, satisfactorily, the adjustment of the Markov Chain Model. The study promoted the mapping of the Ecoclimatic regions of Rio Grande do Sul based on description of the minimum temperature sequence and frost risk, thought Markov Chain Model, the use of the multiple linear regression model and Conrad & Pollak Method, thus identifying the probability dates of occurrence of the first and the last frost of the year, to help in the planning and epoch of cultivation